As described in U.S. Pat. No. 4,315,042 issued to Donald J. Spigarelli on Feb. 9, 1982, incorporated herein by reference and assigned to the assignee hereof, there have been several techniques of "solder leveling" which have been developed for obtaining uniformly thick coatings of solder, on the order of 0.0003 to 0.0006 inches, on parts such as printed wiring boards. As noted in this patent, centrifugal forces have been utilized to remove excess solder. A second technique is to use hot oil jets with the hot oil being projected onto the surfaces of the part which has been immersed in molten solder to remove the excess solder in a controlled fashion. Hot air has also been utilized to blow off excess solder. As noted in this patent there are various problems with the above-mentioned techniques, whereas the above-mentioned patent describes a simplified method and apparatus for solder leveling. In the Spigarelli patent the method of solder leveling described involves the maintenance of a zone of hot saturated inert vapor at the temperature of molten solder above a molten bath of solder. A part to be treated is then moved through the vapor at a rate such that it will become uniformly heated to temperatures approaching that of molten solder prior to immersion therein. After initial heating the part is immersed in the molten solder and is withdrawn from the molten solder through the saturated vapor to the ambient atmosphere. While withdrawing the part, streams of hot high density fluid are projected onto the surfaces of the part in directions opposite to the direction of withdrawal to sweep the surfaces of the excess solder. In this patent the part is vertically lowered through the hot saturated inert vapor into a solder pot and is then removed vertically. While this technique works adequately in a large variety of situations, solder sag can occur due to the vertical orientation of the board as it is withdrawn from the solder pot. While solder sag may not inherently present problems in the electrical operation of the board, solder sag is oftentimes unsightly.
Moreover, the solder removal technique in the aforementioned patent describes movement of the board in a vertical direction which, for production line purposes, requires right angle changes in direction of the board for presolder and postsolder processing. This change in direction of the flow of the product requires complicated mechanical repositioning apparatus and is thus somewhat undesirable for high volume production lines.
Additionally, the Spigarelli patent describes fluid jets angled 15.degree.-40.degree. with respect to the board surface to give a glancing flow for solder removal. While these angles work satisfactorily in most instances, clearing of "thru-holes" or "via holes" in the board is an occasional problem. Moreover, while this patent specifies pressures of 5-40 lbs. per square inch, the energy delivered by the liquid stream depends on the nozzle dimensions and flow rate. While acceptable leveling occurs at the above pressures, a maximally efficient system requires analysis of the energy delivered to the board. Finally, for vertical board transport, the solder films on either side of the board are equal in thickness which makes differential thickness control difficult.